Polyphase DC motors, and more particularly three-phase DC motors of the brushless, sensorless type, are widely used in computer system disk drives, such as floppy disk, hard disk, or CD ROM drives, as well as in other applications. Three-phase DC motors may be modeled as having a stator with three coils connected in a wye configuration. A conventional three-phase DC motor is shown schematically in FIG. 1 including a rotor 1 situated inside three coils 2, 3, and 4. The rotor includes a N-S magnetic set 5 and rotates as the coils 2, 3, and 4 are provided with current in a sequence.
Typically a larger number of stator coils are employed with multiple motor poles. In conventional applications, eight pole motors are used having twelve stator windings and four N-S magnetic sets on a rotor, resulting in four electrical cycles per revolution of the rotor. The twelve stator windings, or coils, are modeled as four groups of coils, each group being arranged as a set of three wye connected coils. Each of the three coils in the wye configuration is joined at one end to a common node, or a center tap, and an opposite end of each coil is connected between a high side driving transistor and a low side driving transistor. The driving transistors are typically n-channel MOS transistors, although other types of transistors may be used. The center tap may be left unconnected, or it may be connected to a controlled voltage source.
A three-phase DC motor is typically operated in a bipolar mode which can be summarized as follows. The three wye connected coils receive current in a sequence to drive the rotor. In each portion of the sequence a current path is established through two of the three coils. The third coil in the wye configuration is left floating, or, in other words, no current is permitted to flow through the third coil.
Current flow through the coils is controlled using the driving transistors. Current flows through a conducting high side driving transistor and its associated coil, through the center tap, and then through a second coil and its conducting low side driving transistor. The sequence of current paths is chosen so that, as the current path is changed, one of the coils in the current path is switched to a floating condition, and a previously floating coil is switched into the current path. In the wye configuration of three coils a total of six different current paths, called phases, are available to drive the rotor. The sequence is also chosen to ensure that when a previously floating coil is switched into the current path, the current flowing in the previously conducting coil will flow in the same direction in the newly conducting coil.
A commutation occurs each time the phase is changed and the position of the rotor at that moment is a commutation point. In the sequence of phases defined above, six different commutation events occur for each electrical cycle in the three-phase DC motor. One electrical cycle corresponds to one full rotation of the rotor.
When the rotor in the motor is rotating, the exact angular position of the rotor is typically ascertained by monitoring a back EMF signal, also called a BEMF signal, which is the EMF induced in the floating coil by the rotating magnetic field of the rotor. The BEMF signal in the floating coil is sinusoidal in nature, and crosses the voltage of the center tap at regular intervals. Conventionally, the commutation points are chosen in relation to the moments in which the BEMF signal crosses the center tap voltage, also called zero crossing points.
To ensure a smooth start-up, the position of the rotor must be known in order to correctly choose the coils for an initial phase. When the motor is starting from a rest position there is no BEMF signal from which the position of the rotor can be determined. Therefore, an alternative method of determining the position of the rotor prior to start-up of the motor is needed.
The choice of an initial phase to start the motor is important in determining the smoothness and speed of the start-up for several reasons. A slight backward rotation may occur in response to an initial current if the wrong start phase is selected. The back rotation may be relatively substantial if the initial phase is incorrect. The back rotation results in small oscillations in the motor which are acceptable in some systems but are undesirable in a high-speed disk drive system. An excessive back rotation may also damage read heads in the disk drive system. If the initial selected phase is too far forward, it moves the rotor in the forward direction, but the step forward between the rotor and the initial phase is so large that oscillations may result.
It is therefore desirable to know the exact position of the rotor prior to start-up to ensure a smooth and fast transition from at rest to full rotational speed. There are several conventional methods for starting a polyphase DC motor. In a method known as align and go, a selected phase is energized regardless of the position of the rotor. The rotor is pulled toward the selected phase and held there to provide a known starting position. The disadvantage of this approach is that the rotor oscillates about the phase for a short time period. The rotor is pulled in a forward rotational direction by driving selected phases once the oscillations have dissipated. The align and go method is electronically simple but may result in a significant back rotation of the rotor and the start-up time of the motor is increased. The align and go method also consumes a substantial amount of power.
An alternative method is an open loop phase stepping method in which the phases are sequentially energized in increments to pull the rotor in a forward direction. The time period between each commutation is large and is gradually reduced as the speed of the rotor increases. The disadvantages of the open loop phase stepping method include the possibility of a substantial back rotation, a long start-up time, and significant power dissipation. A third method for starting a DC motor is an inductive sense method in which the phases of the motor are coupled to a voltage source in a rapid sequence. A time period required for a current to reach a threshold in each phase is measured, and the shortest time period indicates a phase which is closest to the position of the rotor. A disadvantage of the inductive sense method is that a measurement of the time periods in increments of hundreds of nanoseconds is required to reliably detect the phase with the shortest time period. A high frequency clock is necessary to produce a clock signal in hundreds of nanoseconds and creates high frequency noise in a disk drive system which is highly undesirable.